The present invention relates to an assembly robot and, more particularly, to a drilling unit of the assembly robot, mounted on a front end portion of an arm of the assembly robot to be operated as a robot hand during the assembling process, for example, of an aircraft.
Recently, in an assembly line of aircraft, a composite material such as resin impregnated carbon fibers has been used, together with aluminum material, for structural members and outer plates of the aircraft.
In the aircraft assembling process, these structural members are fastened or jointed with each other with a number of fasteners such as round-head rivets or countersunk-head rivets, and high machining precision is required for drilling and countersinking a workpiece. Accordingly, in order to perform such machining, there has been used a special air drill motor such as a so-called "positive feed drill motor" or "spacematic drill motor", provided with an automatic spindle feeding mechanism, or there has been used an assembly robot provided with a drilling unit in which machining processes are preliminarily programmed.
The drilling unit mounted on the assembly robot described above is disclosed in U.S. Pat. No. 4,332,066 to Hailey et al.
The U.S. patent discloses a drilling unit of an assembly robot having a multidegree of freedom and incorporating a remote compliance mechanism. The drilling unit is capable of correcting or compensating for positional errors on the drill side during the drilling operation to secure a predetermined drilling precision. For this purpose, the remote compliance mechanism comprises outer and inner concentric annular guide members disposed in the sam plane. The inner guide member has a peripheral surface formed in the shape of a barrel and is coupled through a resilient material to the outer guide member so as to be swingable. The entire periphery of the outer guide member is supported by a plurality of elastic pins arranged with predetermined angular distances.
However, the remote compliance mechanism of the drilling unit described above is of a single unit type, so that the compliance mechanism has a small capability of compensating for or correcting positional errors and, in a worst case, accumulated errors cause a trouble that a front end of a nose piece of the drilling unit is not inserted into a bush of a drill guide plate or template.
It may be possible for the drilling unit described above to have the front end thereof inserted into the bush of the template by means of the compliance mechanism even if the front end is slightly displaced from the bush. However, since the compliance mechanism is not provided with an unlock mechanism for removing the positioning error, an excessive bending stress may be applied to the drilling unit and the robot body, resulting in damage of the drilling unit, the template or the robot body and in poor drilling and countersinking operation.
Moreover, in the drilling unit of the conventional type described above, the front end of the drilling unit is not fixedly held with respect to the template during the drilling or countersinking operation, so that the drilling arm may be shifted away from the workpiece due to a reaction force acting upon the front end of the drill during the drilling or countersinking operation. Therefore, in the case of countersinking operation, countersinking depth cannot be obtained with high precision. In addition, when the drilling unit is operated with a horizontally directed attitude, a portion of the drilling unit in front of the compliance mechanism is flexed or distorted by the self weight, resulting in the nose piece being not inserted into the template.